The present invention relates to metal matrix composites, particularly SiC-reinforced copper and aluminum.
The coefficient of thermal expansion (CTE) of a material is a factor representative of the degree to which a particular material expands (if a material has a positive CTE) or contracts (if a material has a negative CTE) as it is heated. Most materials have a positive CTE, and expand upon heating.
Materials having low or zero CTEs are useful as structural components in a variety of settings. For example, in fields such as high-power electronics, space optics, precision measurement devices, and the like, where precise measurements, tolerances, positions, and/or shapes of structural components is critical, the use of structural components having low or zero CTE is highly desirable, especially in situations in which the components are exposed to a variety of temperatures. In systems such as these, if structural components have higher CTEs, then as the temperature of the components varies, the components expand or contract, potentially disrupting measurements, settings, relationships between components, etc.
In many cases it is desirable that these components also be highly thermally conductive, such as in electronics thermal management, where high thermal conductivity and a low, tailorable coefficient of thermal expansion (CTE) are needed. For example, in the case of a substrate or a semiconductor chip used in relatively high-power electronics, the chip will generate significant heat and it is desirable that the substrate have high thermal conductivity to remove the heat from the chip.
Composites of metals and CTE-modifying additives find use in electronics thermal management applications. The metal component provides thermal and/or electrical conductivity, and the additive, which can be a ceramic with a CTE much lower than that of the metal, lowers the overall CTE of the composite. Because increasing the ceramic additive content generally decreases the thermal conductivity of the composite, it is desirable to use ceramic additives with CTEs as low as possible to minimize the required volume fraction of additive for a given composite, and thus maximize composite conductivity. Ceramics with negative CTEs thus are particularly attractive, and also provide the opportunity for thermally-conductive metal/ceramic composites with zero isotropic CTE (where the negative CTE of the ceramic offsets the positive CTE of the metal) for applications in precision optics and measurements.
Composites of the general type described above typically have been made by grinding components to fine powders, combining and mixing the powders, and applying pressure to the mixture, heating the mixture, or both. Most typically, a powder mixture is sintered or calcined at relatively high temperature, optionally with pressure, to form a composite. Sintering of copper typically takes place above 800° C. Hot isostatic pressing of copper is normally carried out a temperatures above 600° C.
According to the Lacce “Rule of Mixtures,” the intrinsic physical properties (e.g., thermal conductivity, coefficient of thermal expansion) of a heterogeneous article composed of at least two thoroughly mixed materials tend to vary approximately linearly with respect to the ratio of the volume of one of the materials to the volume of another of the materials. For example, a heterogeneous article composed of a 50—50 volumetric mixture of one material that has a low coefficient of thermal expansion and another material that has a high coefficient of thermal expansion can be expected to have a coefficient of thermal expansion that is the average of the coefficients of thermal expansion of the two materials.
Accordingly, it is an object of the present invention is to provide a method for the formation of metal-matrix composites with combinations of physical and mechanical properties desirable for specific applications.
Other objects and advantages of the invention will be set forth in part in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and attained by means of the instrumentalities and combinations particularly pointed out in the appended claims.